Add #to_anchor String extension

This will convert a string into an URL HREF anchor. Specifically, we anticipate using this for headings in GitHub markdown files to construct links to those headings.

lib/twirp/protoc_plugin/core_ext/string/to_anchor.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class String
+  # Converts the string to an acceptable URL anchor.
+  #
+  # Thw rules for GitHub markdown links are:
+  #   - force lowercase
+  #   - strip punctuation
+  #   - replace spaces with dashes
+  # @return [String] the string converted to an acceptable URL anchor
+  def to_anchor
+    downcase.gsub(/[^a-z0-9_ -]/, "").tr(" ", "-")
+  end
+end

spec/core_ext/string/to_anchor_spec.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+require "twirp/protoc_plugin/core_ext/string/to_anchor"
+
+RSpec.describe String do
+  describe "#to_anchor" do
+    it "converts a string with brackets, numbers, and dates" do
+      expect("[1.1.1] - 2024-05-22".to_anchor).to eq("111---2024-05-22")
+    end
+
+    it "converts a string with mixed case and backticks" do
+      expect("Install the `protoc-gen-twirp_ruby` plugin gem".to_anchor).to eq("install-the-protoc-gen-twirp_ruby-plugin-gem")
+    end
+  end
+end